Electromagnetic transducer



Jan. 9, 1951 WARD 2,537,723

ELECTROMAGNETIC TRANSDUCER Filed Nov. 18, 1947 2 Sheets-Sheet 1 INVENTOR LESLIE RUSSELL WARD ATTYS.

Jan. 9, 1951 L. R. WARD ELECTROMAGNETIC TRANSDUCER 2 Sheets-Sheet 2 Filed Nov. 18, 1947 INYENTOR L EL1E RUSSELL WARD Patented Jan. 9, 1951 Leslie Russell Ward, South Harrow, England,

assignor to Truvox Engineering Company Limited, Wembley, England, a British company Application November 18, 1947, Serial No.

r In Great Britain November 22,1946

This invention relates to electrodynamic loudspeakers or microphones of the moving-coil type, i. e. of the type comprising a coil-winding suspended for movement in a magnetic field and connected with a sound diaphragm, said diaphragm being caused to vibrate to producesound waves by the passage of oscillatory currents through said coil-winding or, conversely, being vibrated by sound waves to cause the generation of oscillatory currents in said coil-winding.

The conventional loudspeakers and microphones of this type usually comprise a magnetic structure including either a permanentor an electro-magnet consisting of several stamped. or cast ferromagnetic members defining an annular magnetic flux-gap in which the above mentioned coil-winding is arranged to operate. The sound diaphragm is usually arranged outside the magnetic structure and is supported in a separate chassis or equivalent supporting element having an annular rim to which the periphery of the sound diaphragm is secured. Such conventional arrangements necessarily have a considerable depth or thickness dimension, the latter being at least equal to the combined thicknesses of the chassis element and the magnetic structure Furthermore such conventional constructions frequently possess a relatively strong magnetic leakage field therearound; this leakage field may cause serious interference with other associated apparatus, for instance, a cathode ray tube in a television receiver.

10 Claims. (01. 17 115.5

The object of the present inventioneis to provide an improved simplified construction for a moving-coil microphone or loudspeaker in which a greatly reduced depth or thickness dimen ion is needed and in which the external magnetic leakage field is materially reduced or even substantially eliminated.

According to the invention the magnetic structure is arranged to serve also as the sound diaphragm supporting chassis and comprises a first member of sheet ferromagnetic material shaped to an arched or cup form, a second member of similar sheet ferromagnetic material adapted to span the arch or cup mouth of said first member and a permanent magnet of solid bar form secured at one end to the inner face of said second member and arranged so as to project at its opposite end into an aperture in the centre of said first member so as to define, in conjunction with said aperture, a magnetic flux-gap for a coil winding carried by the sound diaphragm which is housed between the two members and is secured around its periphery to at least one of said members.

Each of the two members is conveniently cone pressing while the;

stitutecl by a stamping or permanent magnet is preferably in the form of a short cylindrical bar or rod one end only.

In order that the above and other features of readily; understood several embodiments thereof will now be de the invention may be more scribed with reference to the accompanying drawings in which- Fig. 1 is a vertical transverse sectional viewi Fig. 2, a rear elevational elevational view, bodiment of the invention.

Fig. 4 is a sectional detail view,

view and Fig. 3 a front Fig. 5 is a similarly enlarged sectional detail view, taken on the line V Vof Fig. 2, while Fig.

6 is, likewise, an enlarged sectionaldetailiil View;

taken on the line VI -VI of Fig. '2;

Fig. 7 is a fragmentary view, scale, of the junction between magnet centre pole and the front pressing of the magnetic structure, while Figs.,;,,8 and 9 are similar views ofmodifications falling within the scope of the invention.

Figs. wand 11 illustrate two alternative ways-I of securing the solid centre pole permanent mag pressing of the magnetic structure,-

Fig. 12 illustrates for the exposed rear p- Referring more of the drawings, prises a combined diaphragm supporting chassis comprising two members Ill, ll constituted by sheet metal pressings or stampings of ferromagnetic material, e. g.

net by one of its'ends to the associated front of soft iron or mild steel plate, and a short cylindrical barform permanent magnet 12'. The lat-k ter is desirably of one of the well known directional 'aluminium/ nickel/cobalt alloys of very high coercive force.

'The pressing Ill is in the form of a shallow; cone or cup and constitutes the supporting means for the conicalsound diaphragm I 3 of the device. The latter is provided at its ape'it with" the usual cylindrical voice coil former 28 carry 1 ing a coil-winding 29 and is secured around secured in place at;

partly broken away, of one em-i drawn to all enlarged scale taken on the line IV-IV of Fig. 2-

to an enlarged the permanent;

oneform of screening can. end of the magnetic fluxparticularly to Figs. 1,- z and the loudspeaker shown com-v I, magnetic structure and sound.

outer periphery to an annular ring-shaped portion M of the pressing l0. Holes l5 provided in the major conical walled part of the pressing serve as back-of-diaphragm sound outlets.

As shown more clearly in Fig. 4 the apical region of the pressing l0 terminates in a planar region I!) having-a circular aperture 16 in which is secured, as by hanging-over, a tight fitting bushing l1 also of ferromagnetic metal. This. bushing, which is in good magnetic contact with the pressing It], has an accurate cylindrical bore l8 which constitutes the outer pole-face of the magnetic flux-gap for the moving coil winding 29.

The second pressing H serves to span the mouth of the cup-shaped first pressing and has the form of an annular ring. 20 whose interior is spanned by two integral diametral arms 2! which intersect at right angles in the centre of the ring. The resultant quadrantal apertures 22 form front-of-diaphragm sound outlets. The annular ring 26 is provided with a series of tapped holes 23 for receiving screws 24 by which such ring is held clamped in intimate contact with the annular 'rim portion [4 of the pressing I'll 's'o' possible for the this junction.

j'Tlie pressing IE! is further'provided with a flanged-over outer rim 25 adapted to bear upon the outer surface of the ring 2i! with a view to assisting in the accurate location and good magnetic contact of the pressing H] with respect to the pressing ii. The sound diaphragm i3 is provided'with the'u'sual concentric corrugations its outer 'periphery'for imparting the magnetic flux which passes across 21 near to required flexibility of mounting and to clear these the diametral arms 2! are stepped forwardly as shown'at; 26. The plane of the pressing ll, ifi'iaarticular the central region thereof, is one v'v'hichlies normal to the axis of the conical chassis IQ and diaphragm l3 and hence of the bore I-B fcf the annular fiuig gap 38.

-The permanen magnet i2 is mounted end- Wise uponthe rearw ard face pf the pressing H at the intersection point of the diametral arms 2,l "so as", to project rearwardly therefrom with itsi'longitudinal, i. e. 'cen'tral axis, normal to the planeldlf the pressing Il and preciselycoin cidentalfwith th'iaxis of thebore'lil. The rearw'ard'and free end of the magnet, which enters into the bore i8, constitutes the inner pole 31 (if the magnetic flux gap 38. A

f'Th'e magnet I2 is in the form of a solid circu- 1st section rod. It is provided, near to its free rearward end, with areduced diameter region l'll'Z This region commences in'a plane coincident with that of the forward end of the bushing ill and extends forwardly therefrom towards the frontfend' of the magnet, The purpose of this reduced diameter region is to increase the permissib'le tolerance inthe alignment accuracy of the "moving coil former: 2 8 ;w ithin the flux-gap and thereby ,to decrease. the risk of contact of" the moving coil with the. centre pole on account of displacement or misalignment.

The securing of the magnet l2 has to beeffected, at one end only and, when modern and virtually unmachineable steel alloys are used. therefor, without the aid of tapped holes or the like. In the preferred form of the invention this securing is effectedbymeans .of adhesive. As seen more clearly from Figqfl, the front end of the magnet bar is ground truly 1 fiat and. square,

i. 'e,'no rmal to its longitudinalaxis, as at 3|. The 7 central zone 32 of the rearward face of the as to provide as low a reluctance path as mentary mating surfaces.

pressing H is similarly finished truly flat and in the required plane so as to form two comple- These surfaces are then secured together with the aid of a suitable adhesive film 33.

The adhesive employed is preferably one which I can be applied in fluid form and in a cold or substantially unheated state. It should set or harden without the aid of any substantial degree of heating, such as would impair or even destroy the magnetic properties of the magnet. A particularly suitable form ofadhesive process is that described in British Patent No. 517,823 utilising a thermosetting material and wherein each mating surface is first coated with a heathardenable resin such as phenol-formaldehyde condensation product, such coatings being subsequently dried at a temperature below the hardening temperature thereof. A layer of polyamyl acetal powder is then applied between the coated mating surfaces and the parts then held clamped under pressure in a suitable jig while heat to about C. is applied.

- The use of plane mating surfaces is not essential and other forms of somewhat less simple character are shown in Figs. 8 and 9. In Fig. 8, the pressing ll is recessed as shown at 34 to form a socket into which the end of the magnet I2 is arranged to fit snugly while in Fig. 9 the receiving recess is made of frusto-conical form as shown at 35' to receive the correspondingly tapered end 36 of the magnet. In each case 33' denotes the adhesive film.

As an alternative to the use of an adhesive film other fixing methods may be employed. Fig. 10 shows one of these in which the end 4| of the magnet i2 is made a tight force fit into a cylindrical hole'42 in the pressing H. Before insertion the magnet is provided, by grinding, with an annular groove 43 near to its end and, after insertion into the pressing ll, the relatively soft metal of the 'latter' is swaged or pressed into such groove by the application of suitable tool pressure closely around the hole 42. This pressure application may be made on either or both sides of the pressing I I.

" In Fig. 11, the magnet I2 is ground down, as'

at 49, to provide a'spigot-like projection'and a seating shoulder. 'As in the previous example,"

the soft metal of the pressing His swaged or otherwise deformed so' as to gri'pthe virtually unmachineable metal of the magnet. The spigot maybe made tapered if desired. In further alternative methods. of fixing the solid centre pole magnet, the latter may be force-fitted or fuzed in position by an electric arc. f

The sound diaphragm i3 is secured around its outer periphery to the annular ring-shaped portion M by means of adhesive; 'As shown more clearly in'Figs; 5'and 6 the inner region'of the portion-it isrelieved'as shown'at'd'li so asto' netic reluctance of the junction between thepressings. is thereby lowered.

,Thejapical end of the diaphragm l3 and the attached voice coil is accurately centered within the flux-gap 38 by means of a flexible corrugated.

ring 46 which is secured by its inner periphery to the junction point betweensaiddiaphragm and said former 28. The outer periphery of the cen: tering ring is secured by adhesive to a platform 41 formed in the pressing I0. This platform is appropriately spaced forwardly of the bushing ll of the flux-gap and lies in a plane disposed normal to the axis of the flux-gap 38.

To prevent ingress of dust and other foreign bodies into thefluX-gap, a rear sealing cap 48 of felt or similar porous substance is provided. As shown more clearly in Fig. 4, this cap is arranged to be a snug fit over the projecting rim of the bushing I1 and iseither shaped so as to be a snap fit thereon or is secured by adhesive. A spacer 52 disposed between the end of the magnet I2 and the centre of the cap prevents accidental collapse of the latter and consequent fouling of the voice coil. For a similar dust exclud ing reason the centering ring 46 is preferably of imperforate character while the front quadrantal openings 22 of the pressing I I are corresponding;- ly covered by a suitable fabric member 513. The latter is secured by adhesive upon the outer face of the pressing I I and is overlaid around the rim of the latter by the four mounting pads til located between thefixing holes 145.

A terminal strip 53 carries terminal lugs '54 connected by way of the usual ligaments to the two ends of the coil winding 29.

A loudspeaker or microphone constructed in the manner described above with its permanent magnet forming the centre and with the soft-iron or equivalent pressings I0, II forming the equivalent of a spaced magnetic screen therearound, has the advantage of possessing a negligible external magnetic field. It is, in consequence, eminently suitable for use in close proximity to apparatus, such as a cathode ray tube, which may be seriously affected by such a field. In a television receiver, for example, the scanning raster can be seriously distorted or displaced by even a weak magnetic field.

To reduce still further any small field effects that may occur due to leakage fiux around the exposed rear end of the flux-gap 38, the sealing cap 48 may be supplemented or replaced by a member of ferromagnetic material, e. g. fine'iron wire gauze. This cap is arranged to fit, in good magnetic contact, upon the bushing II. Its

depth is such that no substantial short-circuiting of the flux-gap 38 is caused thereby. Alternatively the cap may be made of imperforate iron sheet or, more preferably, of sheet metal provided with a number of small holes as shown at 55 in Fig. 12.

As a means of still further augmenting the degree of magnetic screening provided, the fabric member 56 covering the front pressing I I may be of iron wire gauze and the holes !5 similarly covered.

The invention is not limited to the particular constructions described. For example, in a second embodiment of the invention, the general construction is similar to that already described with the exception that the cylindrical outer flange of the member I ll leads to a further planar fixing plate having a square perimeter. The second member I 2 is also of similar square shape and the two members are held together by clamping bolts. In this embodiment, which is particularly adapted for miniature construction, the diametral arms of the second member are not set forwardly, the requisite diaphragm clearance being provided by an inserted spacing ring.

In anather embodiment the magnetic structure pole of the flux-gap" comprises a planar front member to which the sound diaphragm is secured and a rear member formed as an arched-shape strip secured at each of its ends to the front member. The outer poleface of the magnetic flux-gap may be formed by fianging over the'edge of the hole in the first member while the two members may be secured together by riveting or spot welding if desired.

I claim:

1. In an electro-acoustic transducer of the moving coil type, a combined magnetic flux structure and sound diaphragm supporting chassis comprising a conical shaped first member of thin sheet ferromagnetic metal having an annular shaped peripheral region, a planar apical region disposed parallel to the plane of said annular peripheral region and a central aperture in said apical region, a bushing offerromagnetic metal secured in said hole, a substantially planar second member of ferromagnetic metal disposed in the base plane of said first member and having a similar annular shaped peripheral region in contact with that of said first member, a permanent magnet of solid circular section rod form secured by one end only to the central region of said first member and having its opposite end projecting into the bore of saidbushing so as to define therewith an annular shaped magnetic flux gap, said magnet rod having a reduced diameter region near to but spaced from the end located within said bushing and a conical sound diaphragm having an axially projecting coil form and associated coil winding at its apex, said diaphragm being disposed entirely within the space defined by said first and second members with its peripheral region secured to said first member adjacent to said annular peripheral region thereof and with said coil form and winding disposed within said magnetic flux-gap.

2. A loudspeaker of the moving coil type comprising a first sheet steel pressing of cupped shape and having an annular peripheral region, a central planar region, an annular shaped platform around said central planar region and a circular aperture in said central planar region, a second sheet steel pressing spanning the mouth of said first pressing and having an annular peripheral region in intimate contact with said peripheral region of said first pressing, a central imperforate region and a plurality of sound outlet apertures, a short cylindrical bar permanent magnet of aluminum/nickel/cobalt alloy mounted endwise on the inner surface of the central imperforate region of said second pressing with its opposite end projecting into the central aperture of said first steel pressing, a tight-fitting bushing of ferromagnetic metal in said central aperture having a cylindrical bore constituting the outer pole face of an an nular flux gap whose inner pole is formed by the free end of said permanent magnet, a generally conical sound diaphragm having its outer periphery secured by adhesive to said first pressing, an axially projecting voicecoil at its apex and a flexible centering ring extending outwardly from a region near to its apex, said diaphragm being disposed entirely between said first and second pressings with said voice coil located in said annular fiux gap and With said centering ring secured by adhesive to said platform of said first pressing.

3. A loudspeaker as claimed in claim 2 in which said permanent magnet is provided with a reduced diameter region near to but spaced from its free end to allow increased clearance for the operational movement of the voice coil attached to said sound diaphragm.

4. A loudspeaker as claimed in claim 2 Wherein said outer peripheral edge of the diaphragm is received in a recessed or relieved zone of said first pressing to provide for the adjacent unobstructed magnetic contact between the annular peripheral regions of the two members.

5. A. loudspeaker as claimed in claim 2 in which said permanent magnet is secured in position solely by means of an adhesive.

6. A loudspeaker as claimed in claim 2 wherein said permanent magnet is secured in position by means of adhesive comprising a thermosetting resinous material.

7. A loudspeaker as claimed in claim 2 wherein said permanent magnet is secured in position by its insertion into a hole within said second pressing and then displacing the relatively soft metal of the latter so as to grip the inserted magnet portion.

8. A loudspeaker as claimed in claim 2 in which said permanent magnet is secured in position by fusing with the aid of an electric arc.

9. A loudspeaker as claimed in claim 2 which includes a screening cap of ferromagnetic material fitted over the exposed rear end of the magnetic flux-gap.

10. A loudspeaker as claimed in claim 2 which includes a screening cap fitted over the exposed rear end of said magnet flux-gap for preventing ingress of dust and other foreign bodies into said flux-gap.

LESLIE RUSSELL WARD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,808,297 Dempster June 2, 1931 1,951,531 Ringel Mar. 20, 1934 1,986,856 Ringel Jan. 8, 1935 1,987,413 Olson Jan. 8, 1935 2,025,087 Blair Dec. 24, 1935 2,025,127 Rowe Dec. 24, 1935 2,085,721 Warnke June 29, 1937 2,091,011 Phelps Aug. 2'4, 193'! 2,395,166 Collins Feb. 19, 1946 FOREIGN PATENTS Number Country Date 3,283 Australia Apr. 14, 1932 of 1931 

